1. Field of the Invention
The present invention is directed to transition metal polyanion oxides that can be used as alkali ion combined materials and more particularly to lithium-ion intercalating structures that can be used as electrochemical compounds.
2. Description of the Related Art
The published literature contains many references by those skilled in the art to the insulating nature of these compounds, and the limitations on their utility as battery storage materials thereby created. For example, Gaubicher et al. (J. Gaubicher, T. Le Mercier, Y. Chabre, J. Angenault, and M. Quarton, “Li/β-VOPO4: A New 4 V System for Lithium Batteries,” J. Electrochem. Soc., 146[12] 4375-4379 (1999)) comment with respect to the NASICON compounds that “unfortunately, the anionic units tend to isolate the transition elements, which consequently leads to low electronic conductivity.”
In “Approaching Theoretical Capacity of LiFePO4 at Room Temperature at High Rates,” H. Huang, S.-C. Yin and L. F. Nazar, Electrochem. Sol. St. Lett., 4[10] A170-A172 (2001), explain that “however, owing to their very poor conductivity, initial reports indicated that Li+ can only be partially extracted/inserted at room temperature at modest rates.” And, in “Issues and challenges facing rechargeable lithium batteries,” J.-M. Tarascon and M. Armand, Nature, 414, 359-367 (2001), note that with respect to these compounds that “one of the main drawbacks with using these materials is their poor electronic conductivity, and this limitation had to be overcome through various materials processing approaches, including the use of carbon coatings, mechanical grinding or mixing, and low-temperature synthesis routes to obtain tailored particles.”
Proposed solutions to the poor electronic conductivity have typically focused entirely on coating with carbon or adding a significant excess of carbon during synthesis. Coating with carbon has been described by N. Ravet et al. in “Improved iron-based cathode materials,” Abstr. No. 12, ECS Fall meeting, Hawaii, 1999 and by Morcrette et al. in M. Morcrette, C. Wurm, J. Gaubicher, and C. Masquelier, “Polyanionic structures as alternative materials for lithium batteries,” Abstr. No. 93, Li Battery Discussion Meeting, Bordeaux, Archachon, 27 May-1 Jun. 2001. Co-synthesizing with carbon has been discussed by H. Huang et al. at the Univ. of Waterloo and by Yamada et al. at the Electrochemical Society Fall Meeting, San Francisco, Calif., September 2001. However, the addition of carbon as a conductive additive can lower the gravimetric and volumetric capacity of the storage material. In some instances, about 20 wt % carbon is added to the electrode formulation (approximately 30% by volume). This significant volume of carbon does not typically store lithium storage at the potentials at which the polyanion compounds store lithium.
It is therefore clear and widely acknowledged by those skilled in the art that poor electronic conductivity is, firstly, an inherent feature of the lithium-metal-polyanion compounds discussed herein, and secondly, that this inherent feature limits the applicability of the materials in lithium storage applications, including lithium battery electrodes, especially at temperatures near room temperature. While published literature and patents describe the addition of various metal additives to such compounds, they are silent as to whether the critical and enabling property of improved electronic conductivity can be obtained.